1. Field of the Invention
This invention relates to a process for preparing N,N-dialkyl aromatic amines, and more particularly to a process for preparing N,N-dialkyl aromatic amines from an aromatic amine and a lower alcohol.
2. Prior Art
It is well known to produce an N,N-dialkyl aromatic amine from an aromatic amine and a lower alcohol by liquid-phase method or gas-phase method.
With the liquid-phase method, the reaction is conducted in the presence of a liquid catalyst such as sulfuric acid. In order to cause the starting materials to be present in liquid phase for the liquid-phase reaction, in other words, to facilitate the reaction in liquid phase, it is necessary to use as the liquid phase a lower alcohol in excess, usually in at least three times the amount of aromatic amine, and to conduct the reaction at a very high pressure. In fact, "Unit Processes in Organic Synthesis," (1952, published by McGraw Hill), page 830, for example, discloses that the reaction between 1 mole of aniline and 3.2 moles of methanol in the presence of 0.1 mole of sulfuric acid requires a high pressure of 37 to 39 kg/cm.sup.2. Therefore, with the liquid-phase method which requires a high pressure, it is essential to conduct the reaction in a closed reactor which must be highly resistant to pressure and corrosion because a very corrosive mineral acid is used as catalyst. Consequently, the method involves disadvantageous restrictions on the type and material of the apparatus used. The method further has the drawback of necessitating an additional step in which the reaction mixture is neutralized with an alkali to separate the desired product from the liquid phase. This entails another drawback that is becomes almost impossible to regenerate the mineral acid catalyst.
With the gas-phase method, the reaction is effected in gas phase in the presence of a solid acid catalyst of the silica of silica-alumina type. Although this mode of reaction of preferable, the solid acid catalyst, which has very high catalytic activity in the initial stage of reaction, becomes deteriorated in these characteristics as the reaction proceeds. In 20 hours, for example, the catalytic activity lowers to 80% based on the first activity, and it is therefore impossible to continue the reaction for a prolonged period of time. The cause for the deterioration of catalytic activity, while remaining yet to be fully clarified, appears to be the deposition of tar on the solid acid catalyst. However, a method has not been developed for effectively preventing or remedying the deterioration of catalytic activity, and a useful method of regeneration of the catalyst has yet to be explored.